1. Field of the Invention
The invention relates to a dynamoelectric, rotating machine; and more particularly, to an axial airgap, dynamoelectric, rotating machine comprising a rotor assembly and a stator assembly that includes a frontiron section, a backiron section, and a plurality of stator tooth sections.
2. Description of the Prior Art
The electric motor and generator industry is continuously searching for ways to provide dynamoelectric, rotating machines with increased efficiencies and power densities. As used herein, the term “motor” refers to all classes of motoring and generating machines which convert electrical energy to rotational motion and vice versa. Such machines include devices that may alternatively function as motors, generators, and regenerative motors. The term “regenerative motor” is used herein to refer to a device that may be operated as either an electric motor or a generator. A wide variety of motors are known, including permanent magnet, wound field, induction, variable reluctance, switched reluctance, and brush and brushless types. They may be energized directly from a source of direct or alternating current provided by the electric utility grid, batteries, or other alternative source. Alternatively, they may be supplied by current having the requisite waveform that is synthesized using electronic drive circuitry. Rotational energy derived from any mechanical source may drive a generator. The generator's output may be connected directly to a load or conditioned using power electronic circuitry. Optionally, a given machine is connected to a mechanical source that functions as either a source or sink of mechanical energy during different periods in its operation. The machine thus can act as a regenerative motor, e.g. by connection through power conditioning circuitry capable of four-quadrant operation.
Rotating machines ordinarily include a stationary component known as a stator and a rotating component known as a rotor. Adjacent faces of the rotor and stator are separated by a small airgap traversed by magnetic flux linking the rotor and stator. It will be understood by those skilled in the art that a rotating machine may comprise plural, mechanically connected rotors and plural stators. Virtually all rotating machines are conventionally classifiable as being either radial or axial airgap types. A radial airgap type is one in which the rotor and stator are separated radially and the traversing magnetic flux is directed predominantly perpendicular to the axis of rotation of the rotor. In an axial airgap device, the rotor and stator are axially separated and the flux traversal is predominantly parallel to the rotational axis.
Except for certain specialized types, motors and generators generally employ soft magnetic materials of one or more types. By “soft magnetic material” is meant one that is easily and efficiently magnetized and demagnetized. The energy that is inevitably dissipated in a magnetic material during each magnetization cycle is termed hysteresis loss or core loss. The magnitude of hysteresis loss is a function both of the excitation amplitude and frequency. A soft magnetic material further exhibits high permeability and low magnetic coercivity. Motors and generators also include a source of magnetomotive force, which can be provided either by one or more permanent magnets or by additional soft magnetic material encircled by current-carrying windings. By “permanent magnet material,” also called “hard magnetic material,” is meant a magnetic material that has a high magnetic coercivity and strongly retains its magnetization and resists being demagnetized. Depending on the type of motor, the permanent and soft magnetic materials may be disposed either on the rotor or stator.
By far, the preponderance of motors currently produced use as soft magnetic material various grades of electrical or motor steels, which are alloys of Fe with one or more alloying elements, especially including Si, P, C, and Al. Most commonly, Si is a predominant alloying element. While it is generally believed that motors and generators having rotors constructed with advanced permanent magnet material and stators having cores made with advanced, low-loss soft materials, such as amorphous metal, have the potential to provide substantially higher efficiencies and power densities compared to conventional radial airgap motors and generators, there has been little success in building such machines of either axial or radial airgap type. Previous attempts at incorporating amorphous material into conventional radial or axial airgap machines have been largely unsuccessful commercially. Early designs mainly involved substituting the stator and/or rotor with coils or circular laminations of amorphous metal, typically cut with teeth through the internal or external surface. Amorphous metal has unique magnetic and mechanical properties that make it difficult or impossible to directly substitute for ordinary steels in conventionally designed motors.
A number of applications in current technology, including widely diverse areas such as high-speed machine tools, aerospace motors and actuators, and compressor drives, require electrical motors operable at high speeds (i.e., high rpm), many times in excess of 15,000–20,000 rpm, and in some cases up to 100,000 rpm. High speed electric machines are almost always manufactured with low pole counts, lest the magnetic materials in electric machines operating at higher frequencies experience excessive core losses that contribute to inefficient motor design. This is mainly due to the fact that the soft material used in the vast majority of present motors is a silicon-iron alloy (Si—Fe). It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional Si—Fe-based materials causes the material to heat, oftentimes to a point where the device cannot be cooled by any acceptable means.
To date it has proven very difficult to cost effectively provide readily manufacturable electric devices, which take advantage of low-loss materials. Previous attempts to incorporate low-loss materials into conventional machines generally failed, since the early designs typically relied on merely substituting new soft magnetic materials, such as amorphous metal, for conventional alloys, such as silicon-iron, in machine's magnetic cores. The resulting electric machines have sometimes provided increased efficiencies with less loss, but they generally suffer from an unacceptable reduction in power output, and significant increases in cost associated with handling and forming the amorphous metal. As a result, they have not achieved commercial success or market penetration.
However, a further problem arising in electric machines capable of operating at high frequencies and high speeds is heating in the rotor. As the rotor rotates relative to the stator, the rotor magnets experience cyclic differences in permeance coefficient during the course of each rotation, as the rotor magnets alternately pass between alignment with the teeth of the stator core and positions centered in the gaps between the stator teeth. In turn, this variation in permeance results in changing flux within the rotor, inducing eddy currents in accordance with Faraday's law. Those currents in some cases are high enough to cause significant heating in the rotor. The heating, in turn, is likely to cause irreversible loss of magnetization and reduced device output. In extreme cases, the heating may even be severe enough to reduce the lifetime of the rotor magnets or destroy them.
Accordingly, there remains a need in the art for highly efficient electric devices, which take full advantage of the specific characteristics associated with low-loss material, thus eliminating the disadvantages associated with conventional machines. Ideally, an improved machine would provide higher efficiency of conversion between mechanical and electrical energy forms. Improved efficiency in generating machines powered by fossil fuels would concomitantly reduce air pollution. The machine would be smaller, lighter, and satisfy more demanding requirements of torque, power, and speed. Cooling requirements would be reduced. Motors operating from battery power would operate longer for a given charge cycle. For certain applications, axial airgap machines are better suited because of their size and shape and their particular mechanical attributes. Similar improvements in machine properties are sought for both axial and radial airgap devices